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Friday, August 30, 2013

Blurred Lines on the Tree of Life

The line of classification that separates the living from
the non-living is normally drawn just short of viruses. Unlike a simple cell or bacterium, a virus
cannot reproduce or metabolize on its own – but a virus does have a
genome. And so, although excluded from
among the living, it is difficult to include them among the dead. New findings have now made this line of
distinction even more blurry.

In July a team of French and Swedish scientists led by
Jean-Michel Claverie and Chantal Abergel announced the discovery
of a new kind of virus, and they gave it the epic name of ‘Pandoravirus’. To say that the Pandoravirus is simply a
virus would be an understatement, but it can’t be considered a living cell
either. A virologist at the University
of Pittsburg described
the discovery of this virus as being “like finding a Sasquatch”. Indeed, it appears to be the closest thing to
a missing link between the periodic table
and the tree of life. Here is why.

Pandoravirus is a behemoth.

While most viruses are 50 to 200 nanometers in diameter, the
Pandoravirus is up to 1000 nanometers across.
This is large enough to be visible in a microscope, which is how it was
first observed. Being microscopic is normal
for a microbe, but not for a virus. The
same team of scientists announced the
discovery of Mimivirus
(400 nanometers) in 2003, followed by Megavirus
(750 nanometers) in 2010. These two
viruses were the previous record holders for girth in the virus world. In fact, Mimivirus was mistakenly classified
as a type of bacteria for 11
years. This error highlights the
significance of size with these viruses: they are larger than some bacteria, and
the Pandoravirus even measures up to some eukaryotic microorganisms.

Pandoravirus has serious genetic game.

In simplistic terms, viruses are a bundle of genes
packaged inside of a shell of proteins.
So a bigger virus means more space for more genes. Pandoravirus has up to 2,556 genes – in other
words, blueprints for 2,556 different proteins and enzymes. This level of genetic complexity is on par
with some of the more simple living microorganisms that we know of. For comparison to the average virus, the
seasonal flu virus typically has 12 genes,
and HIV has 9.

Pandoravirus is a freak.

Of these 2,556 genes wrapped inside a Pandoravirus, only
7% can be found in any other virus or living organism. The other 93% are unique genes that have
never been seen anywhere before. To
understand how alien this is in genetic terms, consider that about 61%
of the genes in a fruit fly also exist in humans, in some comparable form. Being as how the Pandoravirus appears to have
no kin in the world of viruses or on the tree of microbial life, this raises
many interesting questions about what its origin may have been.

Pandoraviruses are probably everywhere.

In the reported findings, those responsible for the
discovery of Pandoraviruses actually reported the discovery of two viruses that
were similar enough to both be considered part of a ‘Pandoravirus’ family. This would not be inherently interesting or
unusual, except that one was found in a saltwater sediment sample off the coast
of Chile, and the other was found in a freshwater sample taken from a pond in
Australia (where lead scientist Jean-Michel Claverie just happened to be
attending a conference). Claverie summarized
the significance of this serendipity by stating, “The fact that two of them
were found almost simultaneously from very distant locations either indicates
we were incredibly lucky or that they are not rare. They are probably
everywhere.”

All of this is very exciting for microbiologists and virologists who can’t help
but feel as if Pandora has been summoned. These abnormally large viruses were initially discovered by accident. But now that there has been a purposeful
search for them during the past decade, more are sure to be added to the
list. “We still have more crazy things in store that
we expect to be able to publish next year”, stated lead scientist Chantal
Abergel. If this is the tip of a
biological iceberg, then there may be cause to consider reevaluating whether or
not viruses belong in some domain of their own on the tree of life. But more importantly, this demonstrates the
need to view microbial life as a continuum, where the line between viruses and
microorganisms is more blurry than we had thought before (Figure 1). Without a doubt, it will be very interesting
to see what the rest of this iceberg looks like.

Figure 1. The physical and genomic size of the Pandoravirus
is compared to that of the smallest representatives found in each of the three
domains of life. The physical size of the Pandoravirus approaches
Eukaryotic microorganisms, while genomic complexity exceeds that of some
bacteria.